Cluster flies are larger than house flies ranging from 3/8 to 1/2? long, black in color with short yellow hairs on their thorax, a checkered pattern on their abdomen and have large reddish-brown eyes.Adult female cluster flies lay their eggs in soil near the entrance to earthworm burrows. When the larvae hatch they seek out an earthworm host. The larvae will burrow into the earthworm host and consume the host internally until they are ready to pupate. Adult cluster flies emerge from the soil to find mates and lay more eggs. There could be up to 4 generations each year with the adults overwintering in houses and other structures.During the summer, cluster flies could be found on flowers and fruits. They don’t become a nuisance until late summer – fall when they look to overwinter inside. The adults will fly to buildings in the afternoon and rest on sunny areas. As the sun sets, they seek out cracks and other openings into a structure then congregate in attics, wall voids and other out-of-the-way places. When temperatures cool, cluster flies stay dormant but on sunny warm days in the fall and winter they may become active as they are tricked into thinking it is spring. Cluster fly activity inside, at this time, is usually found around windows as they are attempting to leave the home.